I am not sure what you have done to cause this (not enough infrmation supplied), but your computer still boots = good!
I had a lot of good stuff on that distro darn it
One way to solve your problem.
Let it boot.
Plug in an external disk.
Switch to a terminal with<CTRL><ALT><F2>
Login as yourself.
You'll be at a console prompt (sorry, but learn to deal with the basics if you want to be able to rescue this sort of situation).
Copy your essential files to the external disk.
start repairing the problem, once you have your essential personal data safely backed up.
Oh, I tried once before to update my previous distro of Gutsy to Hardy and will never do that again without a clean install.
Yes, a clean install is the way to go (after
you have backed up your old home, then you can reload your needed data files)
Welcome to LQ!